Effects of long-term administration of riboflavin, sodium butyrate or riboflavin 2',3',4',5'- tetrabutyrate ( RTB ) on the activities of renal and hepatic enzymes that catalyze the beta-oxidation of fatty acid were determined in the rat. Feeding of riboflavin or sodium butyrate for 5 weeks had no effect on all the enzymes examined. By contrast, feeding of RTB resulted in an increase in the hepatic activity of 3-ketoacyl-CoA thiolase [EC 2.3.1.16] by 50% of the control level, while the activities of renal 3-ketoacyl-CoA thiolase and of hepatic and renal acyl-CoA synthetase [EC 6.2.1.3] and acyl-CoA dehydrogenase [EC 1.3.99.3] remained unaffected. The increase in hepatic 3-ketoacyl-CoA thiolase activity suggests that prolonged RTB administration results in an increased beta-oxidation of fatty acid in the liver, which may explain the reported reduction in the concentration of tryglyceride in plasma during RTB treatment.